Except for the B757, which is classified as 'heavy' for separation purposes regardless of TOW

For the 757-200, I think it was because of it's wake turbulence.

757-300 exceeds the 255,000 rule so it's always a heavy.

I'd also like to add that although the 757-200 is a "heavy" for separation purposes, the word "heavy" is not added to the end of the callsign. Only the 757-300 flights have "heavy" added to the end, because they are the ones that have a MTOW over 255,000 pounds.

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All A330's and A340's are heavy

Well that's WONDERFUL! Thanks for sharing ... Were you aware that some 757s, all 747s, 767s, 777s are heavy?